A. Field of the Invention
The present invention relates generally to halftoning of images, and, more particularly to digital color halftoning with generalized error diffusion and vector green-noise masks.
B. Description of the Related Art
Digital halftoning is a technique used by binary display devices to create, within the human eye, the illusion of continuous tone. Designed to mimic analog techniques, dot-clustered ordered dithering or amplitude modulated (AM) halftoning produces this illusion by varying the size of round printed dots which are arranged along an ordered grid. When using AM halftoning, the parameters of particular importance are the lines-per-inch (Ipi), i.e., the number of rows/columns of the regular grid and the screen angle or the orientation of the regular grid relative to the horizontal axis. Typically, monochrome screens have an angle of 45xc2x0 since the human visual system is least sensitive to diagonal artifacts.
In color printers, the illusion of continuous shades of color is produced by superimposing the binary halftones of cyan, magenta, yellow and black (CMYK) inks. As the dots of an AM halftone form a regular grid, clustered-dot dithering suffers from xe2x80x9cmoirxc3xa9xe2x80x9dxe2x80x94the secondary interference patterns created by superimposing two or more regular patterns. To minimize the appearance of moirxc3xa9, the screens of cyan, magenta, yellow and black are typically oriented at the angles of 15xc2x0, 75xc2x0, 45xc2x0, and 0xc2x0, creating a pleasant rosette pattern.
The problems of moirxc3xa9 and screen angles are avoided in frequency-modulated halftoning where continuous tone is produced by varying the distance between printed dots and not varying the size. Typically, FM halftones are produced by error diffusion which creates a stochastic arrangement of dots. Besides avoiding moirxc3xa9, FM halftoning, by isolating minority pixels, maximizes the spatial resolution of the printed image relative to the printer. Unfortunately, this distribution also maximizes the perimeter-to-area ratio of printed dots, making FM halftones more susceptible to printer distortions such as dot-gain, and causing the printer halftone to appear darker than the original ratio of white-to-black pixels. In printers with high dot-gain characteristics, AM halftoning may be the preferred technique, with its lower spatial resolution, moirxc3xa9, and perimeter-to-area ratio of its clustered-dots.
An alternative to AM and FM halftoning is disclosed in R. Levien, xe2x80x9cOutput dependent feedback in error diffusion halftoning,xe2x80x9d ISandT""s Eighth International Congress on Advances in Non-Impact Printing Technologies, pp. 280-282 (1992). Error diffusion with output-dependent feedback is an AM-FM hybrid which creates the illusion of continuous tone by producing a stochastic patterning of dot clusters which vary in both their size and in their separation distance. The major advantage of this technique over prior error diffusion schemes, is that by adjusting a single parameter, the output is tunable, that is, capable of creating halftones with large clusters in printers with high dot-gain characteristics and small clusters in printers with low dot-gain characteristics. Error diffusion with output-dependent feedback, therefore, sacrifices halftone visibility for printer robustness.
In U.S. patent application Ser. No. 09/228,573, filed Jan. 11, 1999, which was based upon provisional Serial No. 60/071,649, filed Jan. 16, 1998, the inventors of the present invention disclosed a technique that creates patterns described in terms of their spectral content as green-noise-containing no low or high frequency spectral components. This green-noise model is presented in accordance with a blue-noise model that describes the spectral characteristics of the ideal error-diffused halftone patterns as having no low-frequency content. Furthermore, just as the spectral characteristics of blue-noise generate the blue-noise mask and a binary dither array reduces the computational complexity associated with FM halftoning, the present inventors, using the spatial and spectral characteristics of green-noise, introduced the green-noise mask.
The problem yet to be addressed in the evolution of green-noise halftoning is its application to color. FM halftoning has been studied in great detail with respect to color printing. The techniques introduced range from simply halftoning each color independently, to complex model-based techniques which transform the CMYK color space to alternate spaces, and even using a blue-noise mask for color halftoning.
An object of the invention is to address the problems associated with the related art.
A further object is to provide an apparatus capable of digital color halftoning with generalized error diffusion and vector green-noise masks.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.
To achieve the objects and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention comprises an apparatus and method of color printing using green-noise halftoning, the method comprising the step of: extending error diffusion with output-dependent feedback to cluster pixels of like color and to regulate the clustering of pixels of different colors, wherein the halftoning of different colors is correlated such that the superimposing of different inks is increased or decreased, offering greater control of resulting halftone patterns.
To further achieve the objects, the present invention comprises an apparatus and method of color printing using green-noise halftoning, the method comprising the step of: incorporating a desired correlation between colors to construct a vector green-noise mask, wherein the mask maintains all the desirable attributes of a monochrome mask, such as isotropic and tunable coarseness, while regulating the overlapping of pixels of different colors.
To still further achieve the objects, the present invention comprises an apparatus and method of color printing, comprising the step of: extending error diffusion to regulate the overlapping of different colored pixels, wherein the halftoning of different colors is correlated such that the superimposing of different inks is increased or decreased, offering greater control of the resulting halftone pattern.
To further achieve the objectives, the present invention comprises an apparatus and method of color printing, where {xcfx86i,gi: i=1, 2, . . . , C} is an initial set of empty Mxc3x97N arrays, and C, M and N are integers, the method comprising the steps of: creating a set of Mxc3x97N arrays, { U1, U2, . . . , UC}, of uniformly distributed random numbers such that Ui[m, n]∈(0, 1) is the probability that xcfx86i,gi will become a minority pixel; for each pattern xcfx86i,gi, where the ratio of the total number of minority pixels to the total number of pixels is less than a gray level gi, performing the following substeps: constructing a concentration matrix CMi using a user-defined mapping of, {HLP{circle around (x)}xcfx86i,gi}, the output after filtering xcfx86i,gi with the low-pass filter HLP using circular convolution, locating the majority pixel in xcfx86i,gi with the highest modified probability, and replacing that pixel, xcfx86i, gi[m, n], with a minority pixel, and given the new minority pixel, xcfx86i, gi[m, n], adjusting the probability of each and every majority pixel, xcfx86i, gi[o, p] for l=1, 2, . . . , C, such that: (Ul[o, p])new=(Ul[o, p])oldxc3x97Rl, i(r) where r is the minimum wrap-around distance from the majority pixel xcfx86i, gi[o, p] to the new minority pixel xcfx86i,gi[m, n]; and obtaining the desired color dither pattern given by the set {xcfx86i,gi: i=1, 2, . . . , C} if for all colors i, the ratio of the total number of minority pixels to the total number of pixels in xcfx86i,gi is equal to gi, otherwise, returning to the locating substep.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.